1. Field of the Invention
This invention relates to steering of a vehicle and more particularly to a dual steering system for a material handling vehicle.
2. Description of the Prior Art
Frequently, a heavy duty material handling mechanism, such as a crane or excavator, is mounted from an upperstructure supported by a truck chassis. The truck is capable of being driven over the road or highway under the control of an operator in the main truck cab. The material handling mechanism is supported on the upperstructure which is mounted by a center pin for relative movement with respect to the truck chassis. An operator's cab is provided on the upperstructure to move with the material handling mechanism. During operation at a construction site, an operator in the operator cab can control movement of the truck. U.S. Pat No. 3,599,814 teaches a remote drive mechanism for powering the drive wheels from an auxiliary engine mounted on the upperstructure.
In a prior art machine steering from the truck cab is accomplished through a steering wheel which provides the mechanical input to the steering gearbox. Hydraulic power is provided to the steering gearbox from a power steering pump which is driven by the truck engine. The steering wheel provides mechanical control of the hydraulic power applied to the steering gearbox. The mechanical output of the steering gearbox, through a relay arm, is linked to a steering arm which is connected to the front wheels. Steering from the operator's cab, located on the upperstructure, is accomplished through a steering lever which is mechanically linked to a steering valve. The steering valve receives hydraulic power from a power steering pump located on the upperstructure power unit. The hydraulic output from the steering valve is routed through a center pin to a steering cylinder located on the truck chassis. The steering cylinder can be extended or retracted in response to the output of the steering valve. The output of the steering cylinder is connected to the relay arm which is connected to the steering arm for positioning the front wheels. Steering from the operator's cab can override steering from the truck cab. In this prior art machine the steering system from the truck cab and the steering system from the operator's cab are hydraulically and mechanically independent before the relay arm.